1. Field of the Invention
The present invention relates to an upper support used in a suspension of a vehicle which is provided between a vehicle body and a wheel and includes a shock absorber and a coil spring coaxial with the shock absorber. More particularly, the present invention relates to an upper support in which load bearing rubber members are used in a compression direction to avoid shear.
2. Description of the Related Art
A typical suspension including an upper support, a shock absorber and a coil spring is disclosed in Japanese Utility Model Publication SHO Pat. No. 59-78106. FIG. 2 illustrates a general arrangement of a suspension in a vehicle; FIG. 3 illustrates the upper support disclosed in Japanese Utility Model Publication SHO Pat. No. 59-78106; and FIG. 4 illustrates a load-deformation characteristic of the upper support of FIG. 3.
In FIG. 2, a rear axle beam 51 which supports wheels at both end portions thereof is supported by a vehicle body by means of (a) a suspension arm 52 which is pivotally connected to the vehicle body at one end of suspension arm 52 and is coupled to rear axle beam 51 at another end of suspension arm 52 and (b) a shock absorber 53 and a coil spring 54 provided coaxial with shock absorber 53. The upper end portions of shock absorber 53 and coil spring 54 are coupled via an upper support, for example, an upper support 60 of FIG. 3 to the vehicle body.
As shown in FIG. 3, upper support 60 comprises: a first member 63 including a flange portion 62; a second member 67 including a first cylindrical portion 64 having a small diameter and a second cylindrical portion 65 having a large diameter and a spring seat portion 66; and a third member 68 located inside the first cylindrical portion 64. Upper support 60 further comprises: an inside rubber layer 70 located between and connected by means of vulcanization to second and third members 67 and 68 and contacting a lower retainer 69 at a lower end of inside rubber layer 70; an outside rubber layer 71 located between, and connected by means of vulcanization to first and second members 63 and 67; a rubber seat 72 fixed by means of vulcanization to a lower surface of spring seat portion 66 of second member 67; and a rubber seal 73 connected by means of vulcanization to an outside surface of first member 63.
In upper support 60 of FIG. 3, loads from a coil spring 54 (FIG. 2) are supported, via rubber seat 72, second member 67, outside rubber layer 71 and first member 63, and finally by a vehicle body 61. Loads from a shock absorber 53 are divided into two kind of loads, bound loads caused when an absorber rod moves upward relative to the vehicle body and rebound loads caused when the absorber rod moves downward relative to the vehicle body. A large portion of the bound loads is transmitted from third member 68 to inside rubber layer 70 and from inside rubber layer 70 to second member 67, accompanied with a shear deformation of inside rubber layer 70. A small portion of the bound loads is transmitted from lower retainer 69 via a lower portion of inside rubber layer 70 to second member 67, accompanied with a compression deformation of a lower portion of inside rubber layer 70. The bound loads which have been transmitted to second member 67 are transmitted from second member 67 via outside rubber layer 71 to first member 63, accompanied with shear and compression deformations of outside rubber layer 71, and are finally supported by vehicle body 61. Rebound loads from shock absorber 53 are entirely transmitted from third member 68 via inside rubber layer 70 to second member 67, accompanied with a shear deformation of inside rubber layer 70. The rebound loads which have been transmitted to second member 67 are further transmitted from second member 67 via outside rubber layer 71 to first member 63, accompanied with shear and tension-compression deformations of outside rubber layer 71 and are finally supported by vehicle body 61. Because inside rubber layer 70 is used in shear and large tensile stresses remain at a radially inner portion of inside rubber layer 70 due to the vulcanization of inside rubber layer 70 to third member 68, and because rubber is much weaker against repeated loads of shear and tension than against repeated loads of compression, third member 68 is radially expanded after vulcanization to put the radially inner portion of inside rubber layer 70 in a pre-compressed state.
FIG. 4 illustrates a characteristic A of upper support 60 against loads from a coil spring and a characteristic B of upper support 60 against loads from a shock absorber. As will be understood from FIG. 4, a spring constant of an assembly of inside and outside rubber layers 70 and 71 against the loads from the absorber rod which usually have small vibrational amplitudes is set small to provide a soft cushion, while a spring constant of outside rubber layer 71 against the loads from the coil spring which usually have large vibrational amplitudes is set large to suppress a noise. The small spring constant of the assembly of inside and outside rubber layers 70 and 71 is mainly obtained through a shear deformation of inside rubber layer 70, because spring constants of rubber decreases in the order of compression, tension and shear.
However, there are the following drawbacks in a prior art upper support, such as upper support 60 of FIG. 3.
Firstly, durability of upper support 60 is not great because inside rubber layer 70 of upper support 60 is used in shear and rubber is very weak against shear.
Secondly, a load-deformation characteristic of inside rubber layer 70 in an axial direction receives a limitation in designing the characteristic of inside rubber layer 70 sufficiently soft, because a large axial deformation of inside rubber layer 70 due to shear deformation deteriorates durability of inside rubber layer 70. This means that soft cushion obtained through shear deformation of inside rubber layer 70 and durability of upper support 60 are not compatible with each other in the prior art upper support.
Thirdly, to maintain a necessary durability for upper support 60, it is necessary to use a considerably hard rubber for inside rubber layer 70 which is effective to suppress an axial shear deformation amount of inside rubber layer 70 to a small one. However, the hard rubber increases a ratio of a dynamic spring constant to a static spring, constant of rubber, and damping of rubber acts greatly in a dynamic deformation. This means a dynamic load-deformation characteristic of upper support 60 becomes hard to a considerably great extent, which decreases soft cushion of upper support 60 and noise suppression effect of upper support 60.
Fourthly, since third member 68 must be radially expanded after vulcanization between inside rubber layer 70 and third member 68 to give pre-compression to the radially inner portion of inside rubber layer where tensile stresses due to a shrinkage after the vulcanization have been caused, the expansion process is added to a manufacturing process of upper support 60, which increases manufacturing costs of upper support 60.